1. Field of the Invention
The present invention relates to electronic sensors, and more particularly to sensors utilizing ultra-wideband transmitters and receivers in narrow field applications.
2. Description of Related Art
One type of narrow field sensor can be referred to as a "trip wire." A mechanical "trip wire" detects motion in or through a narrow field when pressure is applied to a trip wire placed across a path in the narrow field. Non-mechanical "trip wires," such as those formed with electromagnetic radiation or ultrasound, also exist. Laser, infrared, and ultrasound "trip wires" detect motion in or through a narrow field by sensing a disruption in the narrow field across a path. Some examples of "trip wire" applications include using it for game trapping, conveyor belt counting, people counting, and safety sensing for automatic doors. Thus, a "trip wire" sensor can be used to activate a net for trapping game, to count objects on a conveyor belt, count people at a doorway entrance, or to prevent an open automatic door from closing when people are in the vicinity of the doorway.
Common electronic narrow field sensors are based on laser, ultrasound, or infrared techniques. Laser or infrared beam-interruption sensors can be focused in a narrow field. However, visibility between the two ends of the field is required and anything that affects this visibility may impair the sensor. Thus, interference from sunlight, dirt, or snow prevents the proper operation of this sensor. Infrared sensors also have no range adjustments. Similarly, optical and ultrasound sensors suffer from blockage by dirt, rain, snow, or any other material. Sunlight will also provide harmful interference for optical sensors.
The transducers in optical, ultrasound, and magnetic sensors are also expensive to manufacture and thus, make them less attractive for commercial purposes. Other sensors are also not commercially feasible because they drain too much power.
FM-CW and Doppler radar sensors, which might be applied as trip wires, also suffer limitations: low material penetration due to the higher frequencies employed, microphonics caused by the use of short radar wavelengths combined with audio frequency processing, frequency crowding, and poor short-range operation.
Although directional antennas with narrow beamwidths may provide an adequately narrow field or trip wire, they are too expensive in a compact package. Furthermore, at some frequencies, narrow beamwidths are almost impossible to produce in a compact antenna without increasing its complexity. The low frequency microwave band is one example. Thus, at this band, material penetration is available but a compact and practically available narrow beamwidth antenna is not.
Another limitation of these sensors is that, for image processing applications, certain characteristics of the object cannot be obtained. For example, the thickness or density of an object at any desired region of the object cannot be determined.
An effective and commercially practical sensor should be inexpensive to manufacture, drain as little power as possible, last a long time, and be durable and impervious to harsh environmental conditions such as dirt, rain, snow, and sunlight. Furthermore, sensors should be able to use signals that can penetrate certain material without damaging the material. Such material penetration allows users to install these sensors behind wood, concrete, or other objects for security purposes and still permit the sensor to function. The compact size of sensors is also desirable to reduce cost, reduce complexity, and provide ease of installation. In other applications, motion sensors that are insensitive to movement of the sensors themselves are particularly valuable when these sensors are placed on moving objects.